Power supply units are required in data viewing devices in order to convert the a.c. voltage of supply mains to d.c. voltages suitable for the operation of a data viewing device. It is already known to provide conventional power supply units in data viewing devices, which normally contain a mains transformer, a rectifier circuit and a regulated adjusting element. A power supply unit of this type is described, for example, in the German Published Application No. 12 94 527. This power supply unit contains a mains transformer and on the secondary side a switching regulator is provided in order to stabilize the d.c. voltage emitter to a consumer. The switching frequency of the switching regulator is here determined by timing pulses emitter by the consumer. These power supply units have the disadvantage, however, that the mains transformer is relatively large and heavy, their efficiency is relatively poor, and a high outlay is required for cooling the adjusting element.
When such a power supply unit is employed in data viewing devices, magnetic fields produced as stray fields, in particular by the mains transformer and by chokes in the power supply unit, with a frequency of, e.g., 50 Hz corresponding to the mains frequency are input coupled into the deflecting coil or directly onto the electron beam of a cathode ray tube provided in the data viewing device.
In the event of a slight difference between the image frequency of the data viewing device and the mains frequency, a beat occurs which is superimposed upon the characters on the screen of the cathode ray tube and whose frequency is equal to the difference between the image frequency and the mains frequency. This beat is clearly visible and is disturbing to the observer.
The image frequency is determined by the repetition frequency of timing pulses which are produced in a pulse generator arranged in the data viewing device. The pulse generator contains a quartz crystal which determines the repetition frequency of the timing pulses. For example, for the formation of the characters and the construction of the image on the screen, the quartz crystal emits timing pulses having a duration of 7.9 .mu.sec. If, for example, 25 lines each comprising 80 characters are represented on the screen, and 16 timing pulses are in each case required for the return of the electron beam, there is an image frequency of 52.9 Hz and the beat has a frequency of 2.9 Hz.
In the event of an image frequency which is considerably greater than the mains frequency and, for example, is 300 Hz, the super position of the magnetic fields results in a haziness of the electron beam on the screen which is likewise disturbing. In both cases, although the super positions can be reduced by screening the cathode ray tube and/or the mains transformer and the chokes, they cannot be eliminated.
It is also known to use switching power supply units. These switching power supply units are already generally known and described for example, in a publication by R. Ranfft: "Schaltnetzteile mit schnellen Transistoren hoher Sperrspannung" in "Valve-Berichte" Edition 18, Vol. 1/2, pages 143 to 153 and in a "Valvo-Brief" of 19.9.1974. In the switching power supply units, the mains voltage is first rectified and filtered. The d.c. voltage obtained in this way is conducted to a regulated d.c. voltage converter. Here an electronic switch in combination with a transformer, secondary rectification and a filter circuit, and together with a magnetic store and a charging capacitor facilitates physically separate transmisson with low losses. When such a switching power supply unit is used with a switching frequency of, e.g., 30 kHz, in the data viewing device although no beat occurs in the characters, the position of the characters on the screen is altered by magnetic fields picked up from the switching power supply unit and the characters quiver about their prescribed positions as soon as the image frequency is asynchronous to the switching frequency of the switching power supply unit. In addition, the fact that the magnetic fields are picked up causes the electron beam to be modulated in its intensity, and the characters on the screen change in brightness so that a flickering of the characters occurs.